This proposal is concerned with translational control in the colonial green alga Volvox carteri. In this alga, the initial phases of development (embryogenesis) occur in the light, but most of the cell divisions in the cleavage stage occur during a subsequent dark period. After cleavage, the newly formed juvenile colony inverts and enters a resting phase until the next light period. The final phases of differentiation are then completed rapidly upon reillumination. Most proteins are not synthesized during the dark phase, despite the presence of a normal population of mRNAs. Thus the final stages of development appear to be synchronized by global translational repression. To study the mechanism of translational repression, the investigator proposes to isolated and characterize mutants defective in translational repression. These mutants will be used in genetic and biochemical studies of the translational control pathway. In a second specific aim, he will test the hypothesis that translational repression involves a change in the phosphorylation state of one or more translation factors, using antibodies to wheat germ translation factors characterized in Karen Browning's laboratory at the University of Texas. A third aim deals with additional experiments designed to investigate other potential regulatory mechanisms, including binding of proteins to the 5' leader regions of mRNAs and possible changes in polyadenylation. Finally, subtractive cDNA libraries will be constructed in an effort to isolate mRNAs specifically translated in the light or the dark. These clones will be useful in the binding and polyadenylation studies and may provide a basis for future experiments designed to localize cis regulatory elements by making chimeric gene fusions.